In a receiver designed to recover customer data from a digital data signal transmitted via a transmission medium, it is well known that timing recovery must be employed to correctly identify the appropriate instants (sometimes called the "timing epoch") at which the received signal must be sampled. This is because the frequencies of the transmitter and receiver clocks invariably differ from one another, if only by a very small amount. Over time, this frequency difference, if not compensated for, would cause the received signal to be sampled further and further away from the appropriate time poits, i.e., with an increasingly erroneous timing epoch. As long as the sampling frequency is high enough, provision of an adaptive equalizer within the receiver might compensate for this clock frequency difference (as long as it is not too large) via the coefficient update process. However, this is not an effective long-term solution, because the distribution of coefficient values will eventually become skewed to one end of the coefficient queue, and equalizer performance will degrade sharply.
To deal with this problem, the receiver is conventionally provided with a timing recovery circuit which determines whether the line samples are being formed earlier or later than they should be and, in response, adjusts the phase of a receiver clock in the appropriate direction. This phase adjustment process is referred to as retarding or advancing the receiver timing or, alternatively, as retarding or advancing the sampling phase. The amount by which the receiver clock is advanced or retarded is referred to herein as the timing adjustment increment.
One commonly used timing recovery technique is the so-called envelope-derived timing recovery, disclosed, for example, in the Bell System Technical Journal, Vol. 54, p. 569 et seq, March, 1975. This technique extracts a symbol-rate tone from the received signal and uses the phase of that tone to control receiver timing. An alternative timing recovery technique is referred to as "coefficient tracking," and uses information derived from the equalizer. See, for example, U.S. Pat. No. 4,245,345 issued to R. D. Gitlin et al, Jan. 13, 1981.
During timing recovery using any of the known techniques, advancing or retarding the receiver clock by the timing adjustment increment is perceived by the remaining circuitry within the receiver as a phase error or "jitter" which can lead to degradation in the signal-to-noise ratio or "gain" achieved in the remaining receiver circuits.
In view of the foregoing, it is the broad object of the present invention to improve the performance of a receiver for digital data transmission in which a timing recovery circuit is used to determine appropriate sampling instants for the received signal. In particular, it is desired to remove or reduce the effects of phase error or timing jitter introduced during the timing recovery process.